A quarter of a million civilians still live in the city, effectively under siege since the army aided by Iranian backed militias cut off the last road into rebel districts in early July.

Syrian rebel fighters launched on Sunday a major assault on government-held southwestern parts of Aleppo in the first major drive to regain ground after major losses last week when the army and its allies tightened its siege of opposition-held parts of the northern city.

A rebel military command center that includes the newly formed group Islamist Jabhat Fatah al Sham, the former al-Qaeda linked Nusra Front and Ahrar al Sham said they had taken over army positions in the southwestern government-held parts of the city in the first few hours of launching the battle to break the siege imposed on rebel-held areas.

The Syrian army confirmed on state media that rebels had waged an offensive but said its fighters pushed back insurgents from an airforce artillery base and denied insurgents had captured the Hikma school.

A quarter of a million civilians still live in Aleppo’s opposition-controlled eastern neighborhoods, effectively under siege since the army aided by Iranian backed militias cut off the last road into rebel districts in early July.

The army, backed by allied militia forces and air strikes from Syrian and Russian jets, had taken last week significant ground on the northern edge of the city, around the Castello road which leads out of Aleppo and north toward Turkey.

The army and pro-government forced took full control of the Bani Zeid district, on the southern side of the Castello road and was amassing troops to make new inroads into the rebel-held areas.

The U.K. Observatory for Human Rights said the assault was by far the biggest military campaign waged by the insurgents against government forces assisted by foreign mainly Iranian-backed militias in Aleppo since the escalation in fighting in recent months.

The monitor which tracks violence across Syria said the airforce intensified their bombing of rebel positions in Hay al Rashdein, Bustan al Qasr and other quarters in the city.

Jets also bombed rebel-held Khan Touman in the southern countryside of Aleppo.

Aleppo, Syria’s biggest city before the outbreak of the conflict five years ago, has been divided between government forces and rebels since the summer of 2012.

Seizing control would be the biggest victory for Syrian President Bashar al-Assad in five years of fighting, and demonstrate the dramatic shift of fortunes in his favor since Moscow joined the war on his side last year.

Assad’s government and its Russian allies declared a joint humanitarian operation for the besieged area on Thursday, bombarding it with leaflets telling fighters to surrender and civilians to leave.

But the United Nations has raised misgivings about the plan and U.S. officials have suggested it may be an attempt to depopulate the city - the most important opposition stronghold in the country - so that the army can seize it.